In fiber optic apparatus it may be necessary to anchor one end of an optical fiber in a fixed position. For example, the end of an optical fiber would have to be set in a fixed position at the output of a laser or other light projecting or light sensing apparatus. To anchor a fiber in a fixed position, it is generally necessary to lock the fiber at two spaced points, as depicted schematically in FIG. 1. In this depiction, laser module 10 comprises a laser 12 which projects light through fiber 14. Fiber 14 passes through lock units 16 and 18, which are anchored to case 11 of laser module 10. These locks hold the fiber in place at locking points 17 and 19, respectively. In general, the purpose of the first lock 16 is to hold fiber 14 in alignment with laser 12, while the second lock 18 is provided so that any stress on the loose "pigtail" portion 20 of the fiber will not be transmitted to lock 16, or so that the entire laser unit 10 can be hermetically sealed. Locking is achieved at the lock points by securing the fiber, such as by clamping, soldering or gluing, as is well known in the art. In the usual configuration, the locks are collinear and the fiber is locked from all movement.
When the fiber is locked in place, stress is almost invariably imposed on the fiber either initially during the manufacturing process or subsequently during use because of thermal expansion mismatch between the fiber and the laser module support structure over the range of operating temperatures. During manufacture and subsequent use, the fiber and laser module are subject to a range of operating temperatures caused by such factors as changes in ambient temperature, manufacturing processes such as soldering, and heat generated by operation of the laser during use. Such changes in temperature cause the fiber to expand and contract in accordance with its coefficient of thermal expansion, while causing the locks to move together or apart in accordance with the thermal expansion coefficient of the components of the laser module to which the locks are attached. The difference in the expansion coefficients causes the fiber and locks to expand and contract with respect to each other, thus causing stress on the fiber.
For example, in the simplified depiction of FIG. 1 fiber 14 is mounted in a straight line between locking points 17 and 19, which are separated by a distance L. Initially both the length of the fiber and the distance between the locks is L. As the temperature changes, stress on the fiber can be quantified by saying that the distance L+dL between the locks differs from the length L that the fiber would assume were the fiber free at the same temperature. If dL is greater than zero, then the fiber is in tension and subject to immediate or eventual fracture. If dL is less than zero, and the fiber is free to flex between the locks, then the fiber tends to assume a sinusoidal shape, as depicted in FIG. 2.
In FIG. 2, the maximum points of stress M are at the locking points 17 and 19, as well as at the center of the sine wave. The minimum stress is at intermediate inflection points I. The bending of the fiber at the locks can damage the fiber and eventually cause it to break. As an analogy, to break a stick, one could extend part of the stick past a table edge (the lock) and bend down on the cantilever so as to concentrate the stress at the table edge, causing the stick to bend and eventually break. Likewise, bending of the optical fiber at the rigid lock point can damage or break the fiber. Further, inhomogeneities in the solder or glue can enhance the stress concentration at the lock.
Such bending stresses are well known, and various techniques have been used to reduce these stresses. One technique to reduce stress is to use materials for case 11 which match the expansion coefficient to that of the glass optical fiber. However, such materials may be quite expensive and often have poor physical properties over the range of operating temperatures and conditions, and, furthermore, the expansion match is usually imperfect.
Another technique used is to provide a slack in the fiber to allow for expansion and contraction. When the locks are aligned as in FIG. 2, the slack would assume the same sinusoidal shape as results from fiber expansion. As discussed above, this results in maximum bending stress at points M, which is particularly damaging at the lock points. In an attempt to alleviate this stress, FIG. 3 depicts a technique in which the second lock 18 is set at an angle to reduce the stress on fiber 14 at lockpoint 19. However, as the fiber assumes its natural sinusoidal bend, it was found that such a configuration still causes undesirable bending stress on the fiber at lockpoints 17 and 19.
Accordingly, there is a need for a method of mounting a fiber between lockpoints to minimize the stresses to which the fiber is exposed during operational temperature cycling.